Recently, there is an increasing interest in energy storage technology. Batteries have been widely used as energy sources in portable phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are subjects of great interest. Particularly, development of rechargeable secondary batteries is the focus of attention. Recently, research and development into novel electrode and battery that can improve capacity density and specific energy have been made intensively in the field of secondary batteries.
Among currently used secondary batteries, lithium secondary batteries appearing in early 1990's have drive voltage and energy density higher than those of conventional batteries using aqueous electrolytes (such as Ni—MH batteries, Ni—Cd batteries, H2SO4—Pb batteries, etc). For these reasons, lithium secondary batteries are advantageously used. However, such lithium secondary batteries have disadvantages in that organic electrolytes used therein may cause safety-related problems resulting in ignition and explosion of the batteries and that processes for manufacturing such batteries are complicated.
It is very important to evaluate and secure the battery safety. The most important consideration is that batteries should not cause damages to users upon miss-operation of the batteries. For this purpose, safety of batteries is strictly restricted in terms of ignition and combustion in batteries by safety standards. Therefore, many attempts have been made to solve safety-related problems of batteries.
In order to prevent heat emission from batteries, various methods including use of a protection circuit, use of heat occlusion by a separator, etc., have been suggested. However, use of a protection circuit causes limitation in downsizing and cost reduction of a battery pack. A mechanism of heat occlusion by a separator often acts inefficiently, when heat emission is generated rapidly.
Recently, use of organic electrolyte additives has been also suggested to solve the above-mentioned problem. However, safety mechanisms based on electrolyte additives have disadvantages in that calorific values (J) may be varied depending on charging current or internal resistance of a battery and that timing is not uniform. Additionally, a device for interrupting electric current by using the internal pressure of a battery needs a space inside of a battery for housing it, and thus is not preferred in terms of high capacity. Moreover, the above conventional safety means are always followed by degradation of other battery qualities.
Korean Patent Publication Nos. 0326455, 0326457 and 0374010 disclose methods for coating inorganic particles on a cathode active material. However, such methods have a disadvantage in that they cause degradation in battery performance even if the battery safety may be improved, because the inorganic particles as coating agent have no lithium ion conductivity.
Meanwhile, electrochemical devices such as lithium ion batteries have problems related with currently used separator in addition to the above safety problems. For example, currently available lithium ion batteries and lithium ion polymer batteries use polyolefin-based separators in order to prevent short circuit between a cathode and an anode. However, such polyolefin-based separators have a disadvantage in that they can be shrunk into their original sizes by heating at high temperature due to the properties of the materials for separators such as melting of polyolefin-based materials at 200° C. or less, and processing characteristics such as stretching of the materials for controlling pore sizes and porosity. Therefore, when a battery is heated to high temperature by internal/external factors, there is a great possibility of short-circuit between a cathode and an anode caused by shrinking or melting of separators, resulting in accidents such as explosion of a battery caused by emission of electric energy. As a result, it is necessary to provide a separator that does not cause heat shrinking at high temperature.
To solve the above problems related with polyolefin-based separators, many attempts are made to develop an electrolyte using an inorganic material serving as a substitute for a conventional separator. Such electrolytes may be broadly classified into two types. The first type is a solid composite electrolyte obtained by mixing inorganic particles having no lithium ion conductivity with polymers having lithium ion conductivity. However, it is known that such composite electrolytes serving as a substitute for a conventional separator and liquid electrolyte are not advisable, because such composite electrolytes have low ion conductivity compared to liquid electrolytes, the interfacial resistance between the inorganic materials and the polymer is high while they are mixed, such composite electrolytes cannot be easily handled due to the brittleness thereof when an excessive amount of inorganic materials is introduced, and it is difficult to assemble batteries using such composite electrolytes. See, Japanese Laid-Open Patent No. 2003-022707, [“Solid State Ionics”-vol. 158, n. 3, p. 275, (2003)], [“Journal of Power Sources”-vol. 112, n. 1, p. 209, (2002)], [“Electrochimica Acta”-vol. 48, n. 14, p. 2003, (2003)], etc.
The second type is an electrolyte obtained by mixing inorganic particles with a gel polymer electrolyte formed of a polymer and liquid electrolyte. See, U.S. Pat. No. 6,544,689, Japanese Laid-Open Patent Nos. 2002-008724 and 1993-314995, PCT International Publication Nos. WO02/092638and WO00/038263, [“Journal of Electrochemical Society”-v. 147,p. 1251, (2000)], [“Solid State Ionics”-v. 159, n. 1, p. 111, (2003)], [“Journal of Power Sources”-v. 110, n. 1, p. 38,(2002)], [“Electrochimica Acta”-v. 48, n. 3, p. 227 (2002)], etc. However, the polymer used in such electrolytes has poor binding ability so that a great amount of inorganic materials cannot be used. Therefore, inorganic materials are introduced in a relatively small amount compared to the polymer and liquid electrolyte, and thus merely have a supplementary function to assist in lithium ion conduction made by the liquid electrolyte. Further, such gel type polymer electrolytes have low ion conductivity compared to liquid electrolytes, resulting in degradation of battery performance.
Particularly, most attempts made up to date are for developing an inorganic material-containing composite electrolyte in the form of a free standing film. However, it is practically difficult to apply such electrolyte in batteries due to poor mechanical properties such as high brittleness of the film.
In addition, U.S. Pat. No. 6,432,586 discloses a composite film comprising a polyolefin-based separator coated with silica so as to improve the mechanical properties such as brittleness of an organic/inorganic composite film. However, because such films still use a polyolefin-based separator, they have a disadvantage in that it is not possible to obtain a significant improvement in safety including prevention of heat shrinking at high temperature. Additionally, Creavis Gesellschaft (Germany) has developed an organic/inorganic composite separator comprising a non-woven polyester support coated with silica (SiO2) or alumina (Al2O3), etc. However, the Creavis's composite separator cannot provide excellent mechanical properties due to the basic characteristics of non-woven webs. Moreover, because the chemical structure of polyester is fragile to electrochemical reactions, the Creavis's composite separator is expected to have many difficulties in practical use in batteries [“Desalination”-vol. 146, p. 23 (2002)].
Accordingly, there is a continuous need for technological research and development into a separator capable of improving performance and safety of an electrochemical device.